Deciphering the roles of environment and development in the evolution of a Late Triassic assemblage of conodont elements

Abstract

To assess evolutionary processes in deep time, it is essential to understand the roles of development and environment, both recorded through the morphological variability of fossil assemblages. Thanks to their great abundance and the high temporal resolution of their fossil record, conodont elements are ideal to address this issue. In this paper, we present the first quantitative study of a Carnian–Norian (Late Triassic) assemblage of closely related P1 conodont elements. Using geometric morphometrics (landmarks, sliding landmarks, and elliptic Fourier analysis), we explore the main axes of phenotypic variation and relate them to classically used taxonomic characters. We show that some important taxonomic features follow laws of covariation, hence highlighting developmental constraints. Furthermore, the intraspecific variation within all considered species, either Carnian or Norian forms, is similarly restricted, emphasizing, for the first time in conodont P1 elements, a common line of least resistance to evolution, which means that similar intrinsic (developmental) factors were acting on these taxa and likely biased the evolutionary trajectories of all these taxa in a similar way. Because the evolution between Carnian and Norian forms is known to have followed a trajectory that is significantly different from the line of least resistance, strong extrinsic pressures, such as environmental disturbances, were probably at play around the Carnian/Norian boundary to counteract the effects of these intrinsic, developmental constraints.

Patterns of bilateral asymmetry and allometry in Late Devonian Polygnathus conodonts

“…2008; Girard & Renaud 2012; Guenser et al . 2019). The buccal elements are most often found isolated, because of the dislocation of the apparatus during fossilization, and because of the procedure of chemical dissolutions most often used to find them in the sediments.…”

“…2018; Guenser et al . 2019) shedding light on the morphological variation of many genera. The platform elements of Palmatolepis , Icriodus and Ancyrodella have been shown to vary synchronously according to short‐term environmental fluctuations (Renaud & Girard 1999; Girard & Renaud 2008).…”

Evolutionary trends of the genus Ancyrogondolella (Conodonta) and related taxa in the Norian (Late Triassic)

“…Neoteny was mentioned also by Orchard (2018), who emphasized that the low number of denticles on the anterior platform margins in the genus Mockina might be a typical example of this phenomenon. Guenser et al (2019) came to a different conclusion when studying upper Tuvalian and Lacian conodont species by geometric morphometric analyses. They stated that the Carnian an-cestors bear a downward bending lower margin, whereas their (Lower) Norian descendants have a more upward bending lower profile (p. 449 and Fig.…”

Cranial evolution in the extinct Rodrigues Island owl Otus murivorus (Strigidae), associated with unexpected ecological adaptations

“…After Procrustes superimposition using TPS Relw 32 41 , consensus configurations and relative warps were processed and visualised as a Principal Component Analysis (PCA) with PAST 3 42 . In an attempt to assess the contribution of characters caught by both views in the overall morphological variation of the assemblage, a STATIS compromise was built following 43 , 44 .…”